Our studies are directed at determining the arrangement of succinic oxidase, a major segment of the respiratory chain of heart mitochondria. Succinic oxidase as a unit has not been isolated but can be reconstituted from three purified and functionally active fractions, a succinate-ubiquinone reductase (complex II) a reduced ubiquinone cytochrome c reductase (complex III) and cytochrome c oxidase. Each fraction is a multi-component complex of flavin or heme along with non heme iron or copper atoms in association with several different polypeptides. Presently, cross-linking studies are being conducted in which cleavable bifunctional reagents are being used to "map" out the proximities of polypeptides one to another in each isolated complex and as the complex is situated in the inner membrane. Also, labelling experiments are underway in which (35S) diazobenzenesulfonate, a membrane impermeable reagent is being reacted with mitochondria (cytoplasmic surface of the membrane outermost) and submitochondrial particles (matrix surface exposed) in order to determine the orientation of complexes II, III and cytochrome C oxidase in the inner membrane.